A new study has found that fishing in the Chagos Archipelago has been running at 25 times the level recorded in official statistics – and that a political deal set to transfer sovereignty of the islands to Mauritius could put the area’s landmark marine protected area at risk.

The research, published in the Journal of the Indian Ocean Region by scientists from Sea Around Us – Indian Ocean and the Sea Around Us global initiative, reconstructs the full history of fisheries catches in the archipelago from 1950 onwards, revealing a consistent and dramatic gap between what has been extracted from these waters and what has been officially reported.

Located halfway between Tanzania and Indonesia, the Chagos Archipelago hosts the world’s largest coral atoll, with reefs that represent half of all those in good condition across the entire Indian Ocean. In 2010, the archipelago’s exclusive economic zone was declared a no-take marine protected area — one of the most significant ocean conservation designations anywhere on the planet.

The unreported fishing has multiple sources. Since the UK and US established a joint military base on Diego Garcia in the 1970s, personnel have been permitted to fish recreationally, even after the MPA was created. Before that, workers in the coconut plantations that operated across the atolls fished for subsistence.

Neither category has been consistently captured in official figures. Combined, the study estimates average domestic catches of almost 100 tonnes per year.

“But there is more to the story,” said Dirk Zeller, lead author of the study and director of Sea Around Us – Indian Ocean at the University of Western Australia. “Historically, the UK only reported these catches, but prior to the 1970s, people working in the coconut plantations that operated in the atolls since the 1700s fished for subsistence.

“Subsistence fishing, comprised mostly of invertebrates like shrimps and prawns, declined during the transition between the plantations and the military settlement, while recreational fishing peaked in the early 2000s, when military personnel on Diego Garcia increased following the 9/11 attacks in the US. The latter fished mostly tunas, bonitos, and mackerel.”

The study has also reconstructed commercial catches taken by Mauritian distant water fishing fleets, which Britain recognised as having traditional fishing rights in the archipelago from the mid-1960s. Catches from Mauritian vessels peaked at 350 tonnes per year in the mid-1990s before declining sharply after the MPA’s creation in 2010.

“Mauritian commercial fisheries target mostly groupers, sky emperors, and redfishes, but we also found that there has been a presence of foreign distant water fishing vessels in the region since the 1950s, targeting tuna and tuna-like species,” said co-author Roshni Mangar, a Mauritian researcher collaborating with Sea Around Us at the University of British Columbia.

“Foreign industrial catches reached up to 17,000 tonnes per year by the mid-1990s and early 2000s, dominated by Japanese, South Korean, Taiwanese and Soviet vessels, before the MPA reduced them to around 600 tonnes per year. Since then, Taiwanese and Sri Lankan vessels have been recorded fishing illegally within the protected area.

“The limited patrols have inadequate deterrent effects and, unfortunately, these illegal catches, although relatively low, do pose a biodiversity risk to the MPA,” said Zeller. Both sharks and sea cucumbers appear to be priority targets – species that are highly vulnerable to rapid population depletion even under moderate fishing pressure.

The research arrives at a moment of political uncertainty. Under a deal signed in May 2025, the Republic of Mauritius is set to take ownership of the archipelago, receiving £101 million in annual payments in exchange for allowing the UK to retain control of Diego Garcia, subject to parliamentary approval.

Mauritius has indicated it may reopen the MPA to fishing in exchange for revenue from domestic and foreign access – a prospect that raises serious conservation concerns for one of the Indian Ocean’s most ecologically critical environments.

Mangar called for robust governance measures to be put in place once the transfer is finalised. “In addition to patrols, vessel monitoring systems, port controls, regulatory sanctions, and flag-state collaborations should be implemented once the territory’s transfer is finalised, to make sure that the biodiversity values of the Chagos Archipelago are preserved.”

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